The Operon: A Group of Genes Whose Expression is Coordinated by an Operator - PowerPoint PPT Presentation

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The Operon: A Group of Genes Whose Expression is Coordinated by an Operator

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Title: The Operon: A Group of Genes Whose Expression is Coordinated by an Operator


1
The Operon A Group of Genes Whose Expression is
Coordinated by an Operator
  • By Francois Jacob, David Perin, Carmen Sanchez,
    and Jacques Monod

2
Analysis of bacterial systems leads to
conclusions
  • synthesis of some proteins have double genetic
    determinism involving 2 genes with different
    functions
  • one gene is responsible for the structure of the
    protein (structural)
  • one gene is responsible for governing the
    expression of the former through action of a
    repressor (regulator)
  • regulators have a coordinated effect governs
    the expression of several closely linked genes of
    structure

3
To Explain Effect
  • there must be an operator
  • operator adjacent to gene group and controls
    activity of that group
  • operator is sensitive to a repressor, gene
    expression is inhibited by the repressor action
    on the operator

4
Based on an operator hypothesis, certain mutants
can be predicted
  • some mutants affecting the operator would not be
    able to synthesize the linked genes associated
    with the operator, these mutations would look
    like deletions of the linked genes themselves
  • other mutants involving loss of affinity of the
    operator for the repressor would have
    constitutive synthesis of the proteins
  • unlike mutants that inactivation of regulator
    genes, these mutants would be dominant in a
    diploid heterozygote, but would only affect genes
    near mutated operator

5
Metabolism of lactose and ?-galactose in E. coli
seem to correspond to idea of operator
  • y gene ? galatoside permease
  • z gene ? ?-galactosidase, inactive form CZ
  • i gene ? repressor
  • can make diploid bacteria for the genes by
    transferring sex factors (F), having the
    corresponding fragment of the genome (F-lac)

6
Oc mutants are constitutive
7
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8
Conclusions
  • Oo and Oc mutations affect genetic elements not
    expressed by an indepedent cytoplasmic product
  • they dont fit within classical concept of gene
    structure
  • different from mutations affecting the regulator
    gene
  • they DO conform to hypothesis of the operator
  • similar to metabolism of galactose in other
    bacteria

9
The Operon
  • the hypothesis implies that between the classical
    gene and and the entire chromosome, there is an
    intermediate genetic organization
  • this includes units of coordinated expression
    operons
  • these are under the control of a repressor whose
    synthesis is determined by a regulator gene
  • the proteins encoded by a gene sequence under
    control of the operator will not be separately
    induced
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